1. Field of the Invention
This invention relates to an electrophotographic process for producing voltage images by utilizing differences in distribution voltage caused by a change resistance of a photoconductive layer in an electrophotographic photosensitive member.
2. Description of the Prior Art
There are known various electrophotographic processes. The most popular electrophotographic process is a process which comprises charging and imagewise exposure to produce electrostatic images.
In general, electrostatic images are produced by corona discharging to charge the surface of a photosensitive member and imagewise exposing to selectively dissipate the charge at the exposed portions. The electrostatic images are developed with a toner of a polarity opposite to that of the electrostatic images and the developed images are transferred to a receiving paper. Such an electrophotographic process needs wires and a shield case for effecting corona charging and high voltage for causing corona discharging so that it is difficult to obtain a compact apparatus for electrophotography.
On the contrary, some electrophotographic process capable of utilizing compact electrophotographic apparatuses have been proposed. Representative ones are disclosed in Japanese Patent Laid-open Nos. 68238/1973, 150342/1976, 1027/1978, 61534/1979 and 61537/1979. According to these processes, there can be produced voltage images capable of being developed with a charged toner without corona charging. That is, a voltage is applied to a photoconductive layer provided with electrodes to conduct imagewise exposure to cause differences in the distributed voltage between the exposed portions and the unexposed portions resulting in the formation of voltage images.
Photoconductive layers used for the photosensitive member where the voltage images are formed may be composed of the same material as that for photoconductive layers for conventional photosensitive members. Resolution of the formed images depends on the number of electrodes of the photosensitive member and isolated conductors per unit area. Therefore, there is a drawback that a photosensitive member having an area corresponding to the area of images to be copied and minute pattern electrodes and isolated conductors is difficult to manufacture.
According to conventional methods for forming color images using electrophotographic photosensitive members, it is necessary to expose the original color images three times and the exposure is conducted through color filters, in general, red, green and blue filters. In each of the three exposures, toner images are produced by means of a toner having a color which is a complementary color to the color of the filter. For example, a photosensitive member is charged and imagewise exposed through a red filter followed by developing with a cyan toner, and the images thus developed are transferred to an image receiving paper. And then the same procedure is conducted except that the red filter is replaced by a green filter and a blue filter and the cyan toner is replaced by a magenta toner and a yellow toner, respectively.
According to such a conventional electrophotographic method for forming color images, it is necessary to repeat the image forming procedure at least three times separately.
Therefore, toner images having different colors are transferred to a receiving paper in such a way that the toner images of one color overlie those of another color. It is very difficult to overlay the colors in register. It is required to clean completely the photosensitive member each time when one color toner is used. Otherwise, the three colors are mixed disturbing the formation of a sharp color image. For completely cleaning the photosensitive member, a complicated cleaning device is necessary resulting in a large electrophotographic apparatus. In addition, such complete cleaning results in shortening the life of the photosensitive member. Further, it takes a disadvantageously long time to repeat the color image formation three times.